1. Field of Invention
This invention relates to sputter deposition of thin films in a vacuum, specifically to an improved apparatus for depositing compound thin films.
2. Description of Prior Art
The DC magnetron sputter coating process has proven to be an effective and economical thin film coating process for a variety of films and substrates. There are three major types of DC magnetron.
The first is the round disc or semi conical disc magnetron used for coating semiconductors and computer hard discs.
The second, the planar magnetron is widely used for coating architectural glass.
The third is the rotating cylindrical magnetron. U.S. Pat. No. 4,422,916 To McKelvey (1984) discloses a rotating cylindrical cathode having a continuous surface. Such cathodes have been fabricated of metals and metal alloys and silicon. They have been used to advantage for depositing metals, alloys, oxides, and nitrides. The rotating cylindrical magnetron is the most recent development and has an advantage over the other two types. It can utilize a higher proportion of cathode material.
Magnetron sputtering can deposit most metals and some alloys. Compounds such as oxides and nitrides can be made by reactive sputtering whereby a solid cathode is sputtered in the presence of a gas which will combine chemically with the solid cathode material in a desired manner. Some compound materials can be deposited by sputtering cathodes of the desired compounds.
However, DC magnetron sputtering has definite limitations. It cannot economically deposit many compound thin films, especially semiconductors. Such compounds include those requiring combinations of metals with elements which don't exist as a gas or which can't be converted to a gas without toxic or hazardous properties. Gases such as H.sub.2 S, H.sub.2 Te, H.sub.2 Se, AsH.sub.3, and PH.sub.3 fall into this category and can be used to reactively sputter such semiconductor materials as CdS, CdTe, ZnTe, CdSe, CuInSe2, GaAs and InP. These materials, to be of use, require accurate control of elemental ratios. In the process of reactive sputtering a phenomenon known as the hysteresis effect interferes with accurate control of elemental ratios since it causes rapid pressure swings of the gas pressures which can produce the desired ratios of elements. The reason for the pressure swings is that the cathode surface, which starts as a pure metal, becomes coated with the same material that one wishes to deposit. Most compound materials sputter at different rates than their precursor materials. Since a sputter rate difference implies a change in reactive gas usage this rate difference causes the pressure swings. Some workers in the field try to compensate for the pressure swings by using larger vacuum pumps which mask the swings but don't eliminate local pressure changes around the cathode. Others use more complicated control schemes which usually control the process globally, but again, this may not prevent local pressure difference. Local pressure differences of reactive gas around the sputter cathode can cause imperfections in the deposited film.
Some compound materials can be made into solid compound planar cathodes at considerable cost but are still not economically sputtered. Most solid compound cathodes possess such low thermal conductivity that they must be sputtered slowly to prevent melting or cracking. Even when RF sputter power is applied it cannot make up for the lack of adequate thermal conductivity. Rf sputtering is difficult to use in large area deposition as the frequency used (13.56 MHz) can cause spontaneous plasmas to ignite in undesired areas of larger coating chambers. RF power supplies are significantly more expensive than DC supplies, which has a negative economic effect. Whether RF or DC is used, many times the desired elemental ratio incorporated in the solid cathode is not preserved in the final film because of different sputtering rates for each element. In such cases smaller but still significant amounts of the above mentioned toxic gases must be added to the sputtering process to remedy deficiencies in the elemental ratios of the deposited films.
In nearly all cases cathode material purity is a constant problem. While not as important for architectural films, extreme purity is essential for semiconductor films. For this reason, the electronics industry usually uses magnetron sputtering usually for metal conductors on semiconductor devices and for the coating of magnetic hard discs for computer memories but not for deposition of semiconductors themselves.
Cathodes of pure metals, while the easiest to make, are not available for all metals. Metals having low melting temperatures, such as Gallium or Indium, cannot be sputtered rapidly for fear of melting the metal, which causes it to drip, ruining the substrate nearby as well as the cathode itself. Materials which react with the atmosphere, such as Phosphorus, obviously cannot be easily made into cathodes.
Alloys are often difficult since during casting they can form large areas of non-uniform metal ratio. The component metals of the alloy sometimes do not sputter at the same rate. Both of these effects produce non-uniformities in the deposited thin film.
Finally, it should be noted that fabrication of cylindrical cathodes is much more difficult than for planar cathodes. Planar cathodes can be cast or pressed into flat plates and then clamped to the cathode apparatus. Unless the material already exists in tubular form such as for Stainless Steel, Copper, Aluminum, Titanium, and Zinc alloys tubing, then cylindrical cathodes must either be plasma sprayed, which is more expensive and produces less pure material or the cylinder must be electroplated, which is an effective method but available only for some metals or the material must be vacuum cast around the cylindrical cathode base, which again works only for a few metals and is not inexpensive.
From the above one can understand why cathodes of compound materials are nearly unobtainable for cylindrical cathodes.
U.S. Pat. Nos. 4,356,073 to McKelvey (1982) and 4,443,318 to McKelvey (1984) attempt to deposit multiple layers of different materials with a rotatable but not continuously rotating cathode comprised of clamped slabs of different materials wherein a slab of a selected material can be brought into position in the plasma coating zone and deposited onto the substrate. By the nature of the apparatus the multiple materials are deposited in an awkward sequential layered fashion and do not possess the desired uniformity. In order to achieve a compound thin film after the deposition process, extensive heating is needed to diffuse the layers together into a continuum and even then, because of poor elemental ratio control and lack of uniformity, the resulting compound thin films possess inferior characteristics.